[ATM] low voltage heating elements.

[artbianconi@blast.net]

Mike Lindner showed me a neat heater setup for use on the 
secondary. Built from several 1/4 (?) watt carbon resistors, it 
appears to do a satisfactory job. Given enough resistors, you 
could probably replicate the device at a size suitable for the 
primary too.

Several resistors may have the thermal output needed to disperse 
dew but they do so through tangential contact with the back of 
the mirror. I fear that the unbalanced application of localized 
temps may cause distortion. 

I personally prefer a heating element that provides a more 
uniform temp rise across the entire mirror so I purchased a 
flexible wafer type heater where a printed circuit of modest 
resistance is laminated between two layers of silicon rubber. The 
price is quite modest, under ten bucks as I recall for a heater 
sized to the secondary sized to my 15" mirror.

They only come in circular sizes whereas, in this application at 
least, ideally they'd be elliptical. Since the minor diameter of 
a secondary is the limiting factor, I had to be content with a 
mirror that had less heat at the edges of the major diameter. 

Not a lot of area there. Still I chose to minimize the effect by 
machining an aluminum heat sink with the same major and minor 
diameters as the secondary. I then used a modest amount of 
transistor grease between it and the back of the mirror to insure 
even distribution of heat and then secured the heater wafer to 
that. I tested the device by placing it in the freezer for an 
hour. Then I opened the dish washer after a wash, where it 
instantly fogged over. Switched on the current source and watched 
to see how the "dew" was evaporated. I was pleased to see that 
the dew evaporated uniformly which suggests that the aluminum 
heat sink is doing it's job.

The downside of course is that the aluminum takes time to heat up 
which lengthens the time needed to heat the glass. I can factor 
the delay into the rate of change and set it off sooner. My buddy 
did a lot of development work on electronics for Tuttle before 
Tuttle died. In fact it was George Matko who engineered the dew 
cap Tuttle used to sell. This stuff is a snap for George.

Minco has a nice selection of wafer heaters but there are others:

http://www.minco.com/

Clear Skies

Art Bianconi

- - - Previous message- - - 
How many watts do you need?  Most applications only put out a
fraction of a watt to keep the dew off. For cheap heat sources,
resistors of a small value (10-33 ohms) in series and parallel
arrangements will do a fine job of little heat sources. Bob May
http://nav.to/bobmay bobmay@nethere.com NEW!
http://bobmay.astronomy.net

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